All,
I'm a newb to this, and have been messing with a buddy's GeeeTech 3D printer. It has a Sanguinololou board on it, and I wanted to load the Marlin FW. However, having already bricked one 1284, I want to make sure I do this right. Is there a way for me to verify what's already loaded?

Everything I've read talks about removing the ATMega chip and wiring it to a breadboard, then loading the bootloader. I'm guessing that means I can't load it while it's on the Sanguinlolou board. Please forgive the dumb questions.. like I said.. i'm new to this.

You can install a boot-loader fine while its on the SL board, Ive done this more times than I care to count... Just wire in the isp header pins and use those with a avrisp. (these pins are often missing on cheap boards)

Main issue is the "fuses" make sure you get those correct. (SL has a resonator vs a crystal, requires different fuse settings)
You borads.txt contains what fuse setting the arduino ide will set. (Fuses are set when you upload a boot loader from within the arduinio ide) A generic 644p or 1284p chip profile will presume a Crystal.

Second potential issue is that some of the really cheap avrisps do not support chips with large firmwares, ie they do not support 644p or 1284p

Dust,
If I'm understanding your notes correctly, if I connect an LED (BTW, the link in your notes is no longer valid) to the last pins, and I get the 3 pulses, I have a bootloader? I can attach a pic of my Sanguinololou board it it'll help.